In the 6 stroke Crower cycle water is injected for another power stroke
and to cool the engine but CO2 + steam causes an acid build up problem
if the exhaust steam is condensed and recycled.

In conventional 4-stroke CO2 would come from the exhaust that always
remains in the head space.

What they need is a spring mounted piston head -- a few psi retracts
the piston. It bottoms out under the pressure of compression/power
strokes and provides the usual head space and compression ratio. On
the 4th stroke it extends from lack of back pressure and exhausts _all_
the gas and all the CO2 from the cylinder.

Then inject the water on the 5th stroke.

In 2 stroke most of the CO2 comes from the difficulty of precisely
charging the cylinder. The goal is for the entire interface to reach
the exhaust port at the same time. Either too much exhaust remains in
the engine or too much fuel goes out with the exhaust or both.